This invention relates to communication systems and more especially it relates to mobile radio communication systems of the kind wherein an Asynchronous Transfer Mode (ATM) network is used to support a mobile radio communication system.
The principles of ATM operation are well known to those skilled in the art and are described in our copending British patent application 9213373.5, to which attention is hereby directed.
In ATM systems, data is transmitted over a network in small packets or cells, which each consist of 48 octets of user significant data (known as the information field) and 5 octets of network significant data (known as the header). The term user significant data when used herein is intended to include any type of information passing over the network and includes speech and/or signalling. The header of an ATM cell contains a number of fields as follows. Two fields comprise the Virtual Path Identifier (VPI) and Virtual Channel Identifier (VCI) which are used to route cells across the ATM network. Thus, the combination of the VPI and the VCI fields define at each interface connection identity. Another field comprises Header Error Control data (HEC) which is used to detect and correct errors in the header. A further field contains Generic Flow Control data (GFC) which is used to control user access and is not used within the network. Further fields comprise the Payload Type Indicator data (PTI) and Cell Loss Priority data (CLP) which have significance for transmission of data across an ATM network.
It will be appreciated that in order to pass ATM cells from an ATM fixed network to a mobile terminal over a radio network, provision must be made for transmission of at least some of the header data as well as the information field. One method of transporting ATM cells across a radio interface between an ATM fixed network and a mobile cellular radio terminal is to transport the full 53 octet cell. However, as will be appreciated, this imposes a large burden of header information on the mobile radio system, hereinafter called an overhead. This total overhead occupies about 10% of the available bandwidth which might more productively be used to enable more mobile users to be accommodated by the system.